The present invention relates to an automatic focus detecting device in which at the position of focus a focus signal indicative of the fact that the image of an intended object is correctly focused by a photographing lens is produced.
A variety of automatic focusing cameras have been proposed in the field of optical image forming systems in which a correct focus position is automatically detected so that the lens may be moved to the focus position. However, to date, no automatic focusing single-lens reflex camera has been put in practical commercial use. One of the important reasons for this is that a single-lens reflex camera is a high grade camera which is extremely severe in its requirements for focus detecting accuracy. In addition, as the lens of the single-lens reflex camera is relatively heavy, the lens driving mechanism is necessarily large. Furthermore, in order to stop the photographing lens at the focus position, it is necessary to provide a processing circuit which decides whether the lens is currently positioned in front of the focus position or behind it. The required processing circuit for performing this function is rather intricate. These factors make it difficult to manufacture automatic focusing single-lens cameras reflex.
A focus detecting technique is also known in the art in which, similar to a double-image coincidence type range-finder, the amount of relative shift between two optical images is electrically detected. Furthermore, a focus detecting technique is also known in the art in which contrast information of the image of an object to be photographed is extracted and the maximum value of the contrast signal thereof is detected.
The latter method utilizes the fact the optical intensity or power spectrum of each spatial frequency of the image of an object to be photographed becomes a maximum at the time of focalization. More specifically, with this technique a position-series scanning output signal representative of the brightness distribution of the image is obtained by mechanically or electrically scanning the image with a photoelectric conversion element and the signal thus obtained is differentiated to extract high frequency components, namely, the contrast information. After the differentiation signal has been converted into an absolute value waveform, the peak value of the absolute value waveform in a single scanning period is extracted and is employed as a focus signal.
A so-called "automatic focusing camera" can be constructed in which the photographing lens is automatically driven in accordance with the above-described focus signal. In addition, if a signal indicative of the focus position is utilized, then it is also possible to provide an automatic focus indicating device in which a light emitting element such as a lamp is turned on at the position of focus.
Recently, a technique has been proposed in which a self-scanning type photoelectric conversion element is employed as the scanning means. The use of such a self-scanning type photoelectric conversion element makes the scanning section compact. Therefore, the focus indicating device according to this technique can be readily incorporated into the design of a single-lens reflex camera. The signal provided by the self-scanning type photoelectric conversion element becomes a maximum at the position of focus. However, there may be more than one peak value due to the presence of multiple objects within the view of the photographing lens. Therefore, in order to stop the photographing lens at the focus position, it is required to provide some means for deciding whether a peak value is the true maximum or not. For this purpose, heretofore either two photoelectric conversion elements were disposed on the optical axis on either side of the focal plane or a memory circuit was employed for comparison of signals which were provided for two lens extension positions.
Efforts have been made to detect the correct focus position with high accuracy and to extract a single level digital signal which accurately indicates the focus position. However, where an automatic focus indicating device in which a light emitting element which is operated at the position of focus is built into a camera, indication of the focus position with a focus signal of extremely high accuracy leads to other difficulties. That is, pictures are often taken with a camera held in the hands of the photographer. In this case the camera is liable to be shaken by the photographer's hands as a result of which it is rather difficult to precisely focus the lens on the object. Therefore, if the focus signal of extremely high indication accuracy is used, then the display of the signal may turn on and off making it difficult for the photographer to use the camera properly. On the other hand, for pictures taken by a camera which is fixed in position, a high focus indication accuracy is desirable. These two contradictory conditions affect high grade cameras such as for instance single-lens reflex cameras quite significantly. This is an important reason why it is difficult to manufacture an automatic focus indicating camera.
Accordingly, an object of this invention is to provide an automatic focus indicating device in which all of the above-described difficulties have been eliminated and in which a focus, signal having a high focus indication accuracy when a camera is fixed in position but in which the focus signal is not so high in focus accuracy when the camera is held by hand.
An automatic focus indicating camera utilizing the device according to the invention has significant merits in that the focalization position can be readily detected even when pictures are taken with the camera held by hand, and, if the camera is fixed in position, then focalization indication is carried out with high accuracy.
Another object of the invention is to provide a simple focus indicating device in which the contrast signal is converted directly into a sound signal without using an intricate lens driving mechanism so that the position of the photographing lens where the desired peak value becomes a maximum is detected by listening to variations in the frequency or amplitude of the sound signal. The device positively utilizes the characteristic of the sense of hearing with which it is possible to detect extremely small sound variations and the highest frequency of the highest amplitude in such sound variations. Thus, the essential feature of the device according to the invention resides in that it can satisfy the severe conditions of focus detection accuracy which are required for a single-lens reflex camera.